1. Field of the Invention
The present invention relates to expandable thermoplastic resin beads and in particular to expandable resins having good toughness and cushioning properties, good retention of blowing agents and the resulting foamed articles made from such resins.
2. Description of the Prior Art
Generally, it is easy to obtain polystyrene beads having a high expansion ratio. The resulting foamed articles made from such beads have high rigidity and good shape retention, but have the disadvantage in that they are fragile and have poor chemical resistance, oil resistance and thermal stability. Foamed products of polypropylene resin, on the other hand, have better elasticity, chemical resistance, oil resistance and thermal stability than polystyrene foams. A blowing agent impregnated in polypropylene resin beads tends to dissipate rapidly, and it is necessary, therefore, to pre-foam them rapidly after preparation to obtain expanded beads, or to store these expandable resin beads in a container under pressure. Accordingly, such impregnated polypropylene resin beads have disadvantages during storage and transportation and it is generally difficult to obtain polypropylene expandable beads having a high expansion ratio.
When polypropylene and polystyrene are simply mixed in an attempt to obtain a resin having the desirable characteristics of both polymers, a uniform mixture cannot be obtained. A foamed product prepared from such a mixture undergoes phase separation, and does not have an attractive appearance. In many cases the amount of the polyolefin mixed is small, because when the vinyl aromatic polymer is simply physically mixed with the polyolefin, the two resins do not become uniformly mixed and phase separation occurs. As a result, the impregnation of the blowing agent and the expansion ratio become non-uniform, and a uniform foamed article cannot be obtained. Moreover, because of the low polyolefin content, the toughness and elasticity of the resulting foamed product cannot be improved.
It is known to polymerize a vinyl aromatic monomer, such as styrene, within polyethylene particles. U.S. Pat. No. 3,959,189 discloses a process for producing polyethylene resin particles which comprises suspending in an aqueous medium polyethylene resin particles, adding to the suspension 30 to 100% by weight based on the weight of the particles of a styrene monomer and an initiator for polymerizing the monomer, and polymerizing the monomer inside the polyethylene resin particles. The particles can then be made foamable by impregnating them with a blowing agent, preferably after the polymerization of the styrene monomer and the cross-linking of the polyethylene resin. The blowing agent generally is a volatile blowing agent, i.e. aliphatic hydrocarbons such as n-propane, n-butane, iso-butane, n-pentane, iso-pentane, n-hexane and neopentane; or cycloaliphatic hydrocarbons such as cyclobutane and cyclopentane; and halogenated hydrocarbons such as methyl chloride, ethyl chloride, methylene chloride, trichlorofluoromethane, dichlorofluoromethane, dichlorodifluoromethane, chlorodifluoromethane and dichlorotetrafluoroethane, etc. in an amount between 5 to 20% by weight based on the weight of the polyethylene-polystyrene resin particles.
U.S. Pat. No. 4,782,098 discloses foamable interpolymer beads comprising polyphenylene ether resin and a polymerized vinyl aromatic monomer that is polymerized in the presence of a polymerization catalyst to form interpolymerized thermoplastic resin beads. A volatile blowing agent is introduced under pressure into the thermoplastic resin beads.
U.S. Pat. Nos. 4,303,756 and 4,303,757 disclose a process for producing expandable thermoplastic resin beads where a vinyl aromatic monomer is polymerized onto the backbone of the polypropylene; and introducing a blowing agent into the thermoplastic resin beads. The blowing agent is the same as that taught in U.S. Pat. No. 4,782,098.
U.S. Pat. No. 4,429,059 discloses a process for producing foamable polyolefin particles comprising adding a mixture of 3 to 15 parts by weight of a blowing agent and 0.5 to 5 parts by weight of a blowing aid to 100 parts by weight of polyolefin particles. Column 2, lines 63-66, teach that the blowing aid causes the interior of the particles to be in a plastic state to facilitate the foaming of the particles. Representative blowing aids used as a plasticizer are benzene, toluene, xylene, trichlene, perchlene, cyclohexane, carbon tetrachloride, etc.
Similarly, interpolymers of polyolefins and polymers of vinyl aromatic monomers that can be expanded to form foamed articles are disclosed in U.S. Pat. Nos. 4,303,756, 4,303,757, 4,622,347, 4,647,593, 4,692,471, 4,677,134, and 4,666,946 and U.S. Application Publication 2004/0152795.
Although interpolymers of polyolefins and polymers of vinyl aromatic monomers can be expanded to form a material characterized as having good toughness and cushioning properties, the cost of such materials limits their commercial utility.
It is also known that rubber-reinforced polymers of monovinylaromatic compounds, such as styrene, alphamethyl styrene and ring substituted styrenes are desirable for a variety of uses. More particularly, rubber reinforced polymers of styrene having included therein discrete particles of a rubber, for example, polybutadiene, the discrete particles of rubber being dispersed throughout the styrene polymer matrix, can be used in a variety of applications including refrigerator linings, packaging applications, furniture, household appliances and toys. The conventional term for such rubber reinforced polymers is “High Impact Polystyrene” or “HIPS”. The physical characteristics and mechanical properties of HIPS are dependent upon many factors, including the particle size and amount of cross-linking in the rubber particles.
U.S. Pat. No. 4,777,210 discloses a continuous flow process for producing high impact polystyrene and for providing reliable and reproducible methods for varying particle sizes. According to this process, a pre-inversion reactor is utilized to convert a solution of styrene, polystyrene, rubber (such as polybutadiene) and a peroxide catalyst into a high impact polystyrene material.
U.S. Pat. No. 4,144,204 discloses a monovinylaromatic compound modified with rubber where the amount of rubber dissolved in the monomer prior to polymerization was chosen so that the content of the soft component (gel phase) in the impact resistance polymer was at least 28% by weight based on the weight of the impact resistant polymer.
GB 2,153,370 discloses a HIPS material manufactured utilizing a high molecular weight rubber material having a stated molecular mass of at least 300,000, a viscosity greater than or equal to 140 cps; the resulting HIPS containing between 7 and 10% by weight of rubber, and the polymerization being carried out in the presence of alphamethyl styrene dimer or a compound chosen from n-dodecylmercaptan, tertiarydodecylmercaptan, diphenyl 1,3 butadiene, or various other compounds or mixtures thereof. Also, this process was carried out in the presence of cyclohexane and ethylbenzene equal to at least 7% by weight of the total ingredients. In addition, additives including monotriglycerides of stearates from polyethylene waxes were also necessary.
U.S. Pat. No. 5,084,513 discloses a process for the manufacture of stable interpenetrating polymer blend networks that includes the preparation of a gel of a poly(alkylene) polymer by dissolving it in a mixture of one or more organic solvents and one or more vinyl aromatic monomers and subsequently polymerizing the mixture.
Typically, HIPS type materials are less expensive than interpolymers of polyolefins and polymers of vinyl aromatic monomers and have good impact resistance but can be brittle and/or lack toughness and generally can not be expanded to provide articles having the desirable physical properties obtained with interpolymers of polyolefins and polymers of vinyl aromatic monomers.
Thus, there is a need in the art for materials containing elastomeric or rubber type materials and polymers of monovinylaromatic monomers that can be expanded to form articles having toughness and cushioning properties approaching those obtainable using interpolymers of polyolefins and polymers of vinyl aromatic monomers.